CURRENT CHANNELLING IN SQUARE PLATES WITH APPLICATIONS TO MAGNETOMETRIC RESISTIVITY1

The magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low-level, low-frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three-dimensional bodies. We determine the characteristic anomalies associated with square, plate-like conductors, which are excellent models of many base metal mineral deposits. The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half-plane. The dipole and half-plane models are used to bracket the behaviour of plates of finite size. The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations. The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined. The ability of the integral equation method to model plate-like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the observed anomaly.     

ADAPTIVE SIGNAL PROCESSING THROUGH STOCHASTIC APPROXIMATION*

One of the problems in signal processing is estimating the impulse response function of an unknown system. The well-known Wiener filter theory has been a powerful method in attacking this problem. In comparison, the use of stochastic approximation method as an adaptive signal processor is relatively new. This adaptive scheme can often be described by a recursive equation in which the estimated impulse response parameters are adjusted according to the gradient of a predetermined error function. This paper illustrates by means of simple examples the application of stochastic approximation method as a single-channel adaptive processor. Under some conditions the expected value of its weight sequence converges to the corresponding Wiener optimum filter when the least-mean-square error criterion is used.     

Computational models of metasomatic processes

It is possible to calculate quantitative models of chemical transfer by metasomatic processes if rock density values are available with chemical analyses. A unique solution for the magnitude and sign of chemical transfer can be obtained only if the volume change or absolute variation in any chemical constituent can be established. However, even if evidence for volume change and chemical behavior is ambiguous, constraints can be placed on the chemical and physical characteristics of the chemical transfer process by making calculations based on reasonable assumptions.     

Geordnete Deponie von festen Siedlungs- und geeigneten Industrieabfällen

Ohne Zusammenfassung     

The Late Cenozoic Eridanos delta system in the Southern North Sea Basin: a climate signal in sediment supply?

ABSTRACT The Eridanos fluvio‐deltaic system, draining most of north‐western Europe, developed during the Late Cenozoic as a result of simultaneous uplift of the Fennoscandian shield and accelerated subsidence in the North Sea Basin. This seismo‐stratigraphic study aims to reconstruct the large‐scale depositional architecture of the deltaic portion of the basin fill and relate it to external controls. A total of 27 units have been recognized. They comprise over 62×10³ km³ in the Southern North Sea Basin alone, and have an average delta surface area of 28×10³ km², which suggests that the size of the drainage area was about 1.1×10⁶ km². Water depth in the depocentre is seen to decrease systematically over time. This trend is interrupted by a deepening phase between 6.5 and 4.5 Ma that can be correlated with the simultaneous occurrence of increased uplift of the Fennoscandian shield, increased subsidence of the Southern North Sea Basin, and a long‐term eustatic highstand. All these observations point to a tectonic control on long‐term average rates of accommodation and supply. Controls on short‐term variations are inferred from variations in rates of sediment supply and bifurcation of the delta channel network. Both rates were initially low under warm, moist, relatively stable climate conditions. The straight wave‐dominated delta front gradually developed into a lobate fluvial‐dominated delta front. Two high‐amplitude sea‐level falls affected the Pliocene units, which are characterized by widespread delta‐front failures. Changes in relative sea level and climate became more frequent from the late Pliocene onward, as the system experienced the effects of glacial–interglacial transitions. Peaks in sedimentation and bifurcation rates were coeval with cold (glacial) conditions. The positive correlation between rates of supply and bifurcation on the one hand, and climate proxies (pollen and δ¹⁸O records) on the other hand is highly significant. The evidence presented in this study convincingly demonstrates the control of climate on time‐averaged sediment supply and channel‐network characteristics, despite the expected nonuniformity and time lags in system response. The presence of a clearly discernible climate signal in time‐averaged sediment supply illustrates the usefulness of integrated seismo‐stratigraphic studies for basin‐wide analysis of delta evolution on geological time scales.     

Causes of Geochemical Diversity of Carbon Dioxide Water in Crystalline Rock Masses

The problem of the diversity of the geochemical types of carbon dioxide waters (CDW) in petrografically and mineralogically uniform crystalline rock masses is solved with allowance made for the effect of different boundary conditions (physicochemical parameters) on the geochemical effect of interaction in the rock–water system. The formation of the entire geochemical spectrum of CDW in crystalline rock masses is shown to be explicable on the basis of a model of interaction in granite–water systems at different mass ratios of reacting rock (S) and water (L), different temperatures T, and equilibrium concentrations of dissolved CO₂ (P _CO2).     

Reconstruction of the diagenesis of the fluvial-lacustrine- deltaic sandstones and its influence on the reservoir quality evolution-- Evidence from Jurassic and Triassic sandstones, Yanchang Oil Field, Ordos Basin

The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7×10?3 ?m2) than those of the deltaic sandstones (9.8% and 5.8 ×10?3 ?m2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality.